Crankshaft coupling structure for engine

A crankshaft coupling structure used in an engine and coupled between a piston (A) and a crankshaft (B) to enhance the output torque of the engine is disclosed to include connector (3) pivoted to the crankshaft (B), the connector (3) having a radially extended and smoothly arched sliding slot (30), and a coupling rod member (2), the coupling rod (2) member having a top end pivoted to the piston (A) and a bottom end pivotally mounted with a roller assembly (4) coupled to the sliding slot (30) of the connector (3) and movable between two distal ends of the sliding slot (30) upon reciprocating motion of the piston (A).